Photolithography process involves transferring a pattern on a mask onto a photoresist layer applied on a surface of a silicon substrate by means of exposure and then transferring the pattern onto the silicon substrate by processes including development and etching, etc. The photolithography process is a key process in Large-Scale Integrated circuit (LSI) manufacture because it determines feature size of the LSI. A projection lens in a photolithography tool is a key component of the photolithography process.
In recent years, the feature size has been reduced by reducing exposure wavelength, increasing numerical aperture of the projection objective lens, and reducing photolithography process factors continuously. After years of development, the exposure wavelength of the photolithography machine has been reduced from 436 nm (g line), 365 nm (i line), and 248 (KrF) to 193 nm (ArF). Currently, the numerical aperture of the projection objective lens in the dry photolithography machine has reached a limit value of 0.93. The numerical aperture of the projection objective lens in the photolithography machine can be increased to more than 1.3 with assistance of immersion technology. The feature size of a chip than can be implemented by the photolithography process goes downward beyond a 45 nm node.
For the immersive photolithography technology, dielectric with high refractive index, which is typically water, is used to replace air between a bottom surface of the projection objective lens in a conventional photolithography machine and a wafer, in order to reduce the wavelength of exposure light source and increase the numerical aperture of the lens, and thereby increase resolution.
The system wave front aberration is a characteristic index for evaluating performance of the projection objective lens in the photolithography machine. Whether the photolithography machine can achieve by exposure a feature line width of several nanometers largely depends upon whether the system wave front aberration of the projection objective lens can be controlled within a range of several nanometers. It is thus desired to develop technologies and equipments for detecting the system wave front aberration with an accuracy of sub-nanometer order, which can measure the system wave front aberration of the projection objective lens precisely. Then the feature line width that can possibly be achieved is evaluated to provide quantified data to be used in directing ultra-precise assembly and adjustment of the protection objective lens.
Currently, apparatus that can be used for detecting the wave front aberration of the projection objective lens system in the photolithography machine includes Point Diffraction Interferometer, Line Diffraction Interferometer, and Lateral Shearing Interferometer, etc.